The ‘COMA-B’ System for Behaviour Change: Reset of the COM-B

The COM-B System

In 2011, three psychologists, Susan Michie, Maartje M van Stralen and Robert West (MSW, 2011), proposed “a ‘behaviour system’ involving three essential conditions: capability, opportunity, and motivation…This forms the hub of a ‘behaviour change wheel’ (BCW).”

MSW mention two sources for the idea of the COM-B:

“a US consensus meeting of behavioural theorists in 1991 [see this], and a principle of US criminal law dating back many centuries…Under US criminal law, in order to prove that someone is guilty of a crime one has to show three things: means or capability, opportunity, and motive.”

They continue:

“This suggested a potentially elegant way of representing the necessary conditions for a volitional behaviour to occur…We have built on this to add nonvolitional mechanisms involved in motivation (e.g., habits) and to conceptualise causal associations between the components in an interacting system.”

However, the hub of the COM-B is incomplete and doesn’t work  Here I explain why. Screen Shot 2020-04-03 at 10.34.04.png

In the COM-B system, Capability, Opportunity, and Motivation ‘interact’ to generate Behaviour. C, O and M are claimed to be sufficient and necessary conditions for B.

I show below that this claim is incomplete …but first we need some definitions:


MSW give the following definitions:

‘Capability’ is defined as the individual’s psychological and physical capacity to engage in the activity concerned. This includes having the necessary knowledge and skills. In plain language – what I call ‘plang’ – ‘capability’ equates with ‘fit to’. 

‘Motivation’ is defined as all those brain processes that energize and direct behaviour, not just goals and conscious decision-making, e.g. habitual processes, emotional responding, as well as analytical decision-making.  In plang, motivation equates with ‘need to’.  

‘Opportunity’ is defined as all of the factors that ‘lie outside the individual that make the behaviour possible or prompt it’. In plang, opportunity means ‘can do’. 

Robbing a Bank

According to the COM-B, capability and opportunity cause changes in motivation and changes in behaviour.  I refer to that well-know character Joe Blow (pronouns: him/her/their). 

According to the COM-B:

Joe Blow (JB) would (X rob a bank, Y kiss the queen, Z fly to the moon, whatever) if JB is fit to, needs to, and can do X, Y or Z.

Yet this account is plainly incomplete. A key element is missing from the COM-B. JB must want to carry out X, Y or Z.  If JB doesn’t want to, he/she/they simply won’t do it, no matter what.

Imagine the following:

1) JB is fit to rob a bank because he/she/they is physically strong and has a jemmy and a set of tools for breaking open doors and safes – (fit to).

2)  JB is hugely in debt (to a bank, as it happens) so he/she/they need(s) money very badly, and so they have a strong motive to rob a bank – (needs to).

3) JB knows there is a back alley and a back door with an alarm that a friend who works in the bank will leave switched off on any night of their choosing – (can do).

JB ticks all three boxes but  JB chooses not to rob a bank. Why? There could be a million and one  reasons, e.g.  JB believes that he should not rob the bank because:

robbing the bank would be wrong,

it would be risky – i.e if he is found out he would go to prison,

it would look bad in front of the neighbours,

it would upset the bank manager who he drinks beers with in the local pub,

it would be an unreasonable and unfair act , etc.

For a host of different reasons, JB may desperately need money but does not want to rob the bank to get it.

In spite of JB ticking all three of COM-B boxes, the COM-B fails to correctly predict JB’s behaviour. There is a hidden barrier. In multiple situations people do not do something, even something they need to do, because they simply do not want to do it.

Another quite similar individual who ticks all three COM-B boxes might actually proceed to commit the bank robbery. Imagine: Joe Blow has a twin, Les Blow (LB/him/her/their) who lives on the other side of town. JB tells LB about the bank, the back alley and the dodgy security guard,  LB meets all three criteria – LB is fit to, needs to, and can do the bank robbery. Significantly, however, LB has none of the moral and social scruples held by JB and LB proceeds to rob the bank.

The twins act differently under essentially similar circumstances.  JB didn’t want to rob the bank but  LB wanted to – so LB did – revealing a crucial difference between the twins.

The COMA-B Reformulation

I have suggested that the COM-B requires reformulation because there is a crucial process missing. Also four of the so-called ‘interactions’ do not exist and none of the interactions work in both directions.

Only one ‘interaction’ in the COM-B diagram is anywhere near causal.  In order to do something, a person also has to want to do it more than they want not to do it. This is a delicate balancing act that goes on when we make decisions every day of our lives.

The reformulated ‘COMA-B’ is shown below.

(A = ‘Agrees to’, which makes a better acronym than using W for Wants : COMW-B).

amended COM-B model.png

Adding a box for wants, COM-B is converted into COMA-B.  A further change is the removal of arrows for imaginary interactions. In removing arrows, it is necessary to distinguish enablers from causes. Fit-to capability and can-do opportunity are both enablers of need-to motivation .


All together there are four necessary and sufficient conditions for any action X:

  1. needing to do X
  2. having the capability to do X
  3. having the opportunity to do X
  4. wanting to do X



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